Batteries are a familiar part of everyday life. Many of us depend on heavy lead-acid batteries to start our cars, dry cells to operate cameras, small lithium-iodine batteries to power cardiac pacemakers, and tiny lithium-magnesium oxide coin cells to run watches.
One goal for researchers has been to reduce the size and weight of batteries. Smaller, lighter batteries are needed to keep pace with the reduction in sizes of electronic devices. These smaller batteries are produced to provide electrical energy for a variety of different devices and applications where packaging space is a premium. Examples include computer memory chips, microcircuits, subsystems for hearing and medical applications, communications, and high-speed data processing. For example, thin-film lithium micro batteries have been developed for computer memory chips.
A number of processes are used to develop micro batteries. Solid-state processes are used for forming thin-film batteries and these processes help increase the amount of energy that can be stored in the battery per unit weight and volume. Standard sputtering or evaporation techniques have been used to deposit thin-film components of a battery, e.g., current collectors, cathode, electrolyte, anode, and a protective coating. The thin-film lithium cell, for example, is fabricated by depositing successive layers of the cathode, electrolyte, and anode. When sputtering is used, high voltage causes atoms to be ejected from one metal and electrically deposited as a battery layer on another metallic or nonmetallic surface located in a vacuum. When evaporation is used, a filament of a metal to be deposited is heated by electric current in a vacuum chamber, which makes the filament particles travel to a metallic or non-metallic surface to form a battery layer. The result is a battery that is exceptionally thin (e.g., only 6 microns thick, or one-third the thickness of plastic wrap) and typically having surface areas of 0.5 to 10 cm2. However, there is no fundamental limit on these dimensions, either larger or smaller.
The performance characteristics may be determined by the type of cathode material, the area and thickness of the material, and by operating temperature. For applications requiring high discharge rates, crystalline LiCoO2 is an optimum choice, while for low rate applications or those requiring ambient temperature battery fabrications, amorphous LiMn2O4 may be used. Also, inorganic anode material allows the development of lithium-ion batteries with service temperatures to at least 250° C.
Thin-film lithium micro batteries have been found to have remarkably high specific energies and energy densities. These cells have open circuit voltages at full charge of 3.7 to 3.8 volts, the highest voltage currently achieved in a thin-film battery fabrication. These same fabrication processes have been used to create thin-film batteries that can be deposited either directly on a reverse side of a computer memory chip or onto the chip's protective ceramic package. One possible advantage of these solid-state thin-film batteries include high power and energy densities, having the capability of being recharged, being fabricated to virtually any size, and being bonded onto a variety of substrate material surfaces, such as semiconductors, ceramics, and plastics.